Single-crystal, one-dimensional (1-D) nanostructures of palladium are attractive as interconnects for fabricating nanoscale electronic devices. For example, palladium can form reliable and reproducible ohmic contacts with carbon nanotubes (CNTs) because palladium has a relatively high work function and can easily wet the carbon surface. This capability allows one to elucidate the intrinsic properties of CNTs and to maximize the performance of CNT-based devices such as field-effect transistors (FETs). Another important property of Pd is its exceptional sensitivity toward hydrogen. To this end, polycrystalline, mesoscopic wires made of palladium have been utilized for resistance-based detection of hydrogen gas. However, polycrystalline wires containing gaps between adjacent grains often shrink alter initial exposure to hydrogen and may cause random, irreversible changes to the resistance of a sensing device. It should be possible to overcome this problem by switching to single-crystalline palladium nanowires with better controlled characteristics.
One of the simplest ways to generate 1-D nanostructures of metals is to confine their growth within a template. The nanoscale channels in alumina or polycarbonate membranes have been most commonly used for this purpose. Other types of templates include mesoporous silica, cylindrical micelles, and organic block copolymers, as well as the edges or grooves on solid substrates. Although a template-directed synthesis can be very simple and straightforward, it is limited in terms of the quantity of nanostructures that can be produced in each run of synthesis. It often yields polycrystalline nanostructures, which are less valuable for both fundamental study and device fabrication. In addition, the template needs to be removed in a post synthesis step so the metal nanostructures can be harvested and put to use. As a result, it seems to be impractical to rely on template-directed synthesis if one needs single-crystal, 1-D nanostructures of palladium.
Solution-phase growth has received considerable interest for its capability to produce single-crystal nanostructures with high quality. However, it is not easy to grow 1-D nanostructures of palladium in a solution phase. As a face-centered cubic (fcc) metal, palladium has no intrinsic driving force for the growth of anisotropic structures when the seeds are surrounded by an isotropic medium. As dictated by thermodynamics, palladium atoms are expected to nucleate and grow into cuboctahedrons (with a nearly spherical shape) enclosed by a mix of {111} and {100} facets to minimize the total surface energy. This prediction has recently been verified by experimental studies where 8-nm cuboctahedrons were obtained as the major product when a palladium precursor was reduced at a sufficiently fast rate to exclude any kinetic effect. In general, an fcc metal can only be forced to grow into anisotropic nanostructures through the kinetic control. As demonstrated in previous work, triangular and hexagonal thin plates of palladium could be prepared by operating at an extremely slow reduction rate to induce the formation of stacking faults and thus break the cubic symmetry. For such thin plates, the top and bottom faces account for greater than 70% of the surface and are terminated in {111} facets, while the side faces (less than 30% of the surface) are enclosed by {100} and {111} facets. It is evident that one needs to not only break the cubic symmetry but also substantially increase the coverage of {100} and/or {111} facets on the surface to generate nanostructures with 1-D.
A need exists for a solution phase method for forming anisotropic metal nanostructures. The present invention seeks to fulfill this need and provides further related advantages.